1. Technical Field
The present invention relates generally to reacting a fluid with a body, and more particularly to a body for reacting with a fluid.
2. Description of Related Art
Fluids (e.g., gases, liquids and the like) may be passed over or through a variety of substances in order to facilitate a reaction. The reaction of species in the fluid, the removal of species from the fluid, and the conversion of the fluid in some chemical or physical fashion may be enhanced by contacting the fluid with a suitable body. In some cases, reactions occur at the interface between the fluid and a surface of the body, and in such cases, it may be advantageous to maximize this surface area (e.g., by using a porous body). Reactions between a fluid and a body may also be used to modify the body itself.
Many suitable bodies are substantially porous (e.g., over 5%, 10%, 20%, 40%, 60%, 80%, or even over 90% porous). Porosity may be continuous, and in such cases, the fluid may substantially saturate the body. When pushed by a suitable driving force (e.g., pressure, voltage, magnetic field or other gradient in thermodynamic potential) a fluid may be caused to pass through the body. In some cases, a species may be removed from a fluid by passing the fluid through pores that block the passage of the species (e.g., filtration). In other cases, a species may be removed or reacted to form another species (e.g., in heterogeneous catalysis) and/or combine with the body per se (e.g., gettering). A species may also be substantially adsorbed or absorbed by the body.
Many bodies have been fabricated by choosing relatively pure starting materials and mixing them, and increasingly complex bodies require the addition of more starting materials. For example, a catalytic converter substrate may be required to have certain thermal, mechanical, and surface area properties, so might be made of cordierite (Mg2Al4Si5O18), which may be made by mixing MgO, Al2O3 and SiO2 starting materials. Cordierite containing iron (Mg,Fe)2Al4SisO18 may have some improved properties, and might be made by adding FeO or Fe2O3 to the aforementioned mixture. Sintering aids, grain boundary phases, and catalytic species could be similarly added, and improvements to many materials generally entail the addition of further components.
Many useful bodies, particularly bodies for high temperature applications, include several components, and in some cases, additional components may improve properties. For example, mullite (3Al2O3-2SiO2) materials often have high strength at fairly high temperatures (e.g., 1300 C), and U.S. Pat. No. 3,954,672 identifies certain cordierite-mullite compositions (i.e., of increased complexity) that have some improved properties over mullite. As such, materials having generally improved properties in an application may often be more complex than known materials typically used in the application.
Many useful bodies are fabricated from combinations of SiO2, Al2O3, FeO, MgO, CaO, TiO2 and other materials, and often include one or more useful phases (e.g., mullite, cordierite, spinels, perovskites, amorphous), each of which may include several components. Thus, the discovery of improved bodies for a variety of applications might be enhanced by basing those bodies on compositions known to have useful properties, then increasing complexity around these compositions.
Porous bodies may be used for filtration, including without limitation deep bed filtration, wall flow filtration, cake filtration, and other methods. Generally, an appropriate body for use in filtration may be chosen based upon a variety of factors, including required flow rates, viscosity of the fluid, phase assemblage of the fluid (e.g., suspended solids in a liquid, emulsions), concentration of species (to be treated) in the fluid, desired pressure differential (if pressure is driving the fluid through/past a body), temperature of the application, chemical reactivity (or lack thereof) and other factors. Available geometrical and mass constraints may also determine an appropriate filtration method. For example, large “ponds” of deep bed filtration bodies may be used to filter large amounts of wastewater, whereas catalytic removal of contaminants in an automotive exhaust gas stream may require a small, portable body.
In some applications, the mechanical behavior of the body may be important. Often, the driving force used to cause a fluid to pass through or past a body creates a mechanical stress in the body itself, and the body's resistance to this mechanical stress is a requirement in many applications. Some applications require that a body have sufficient mechanical strength to withstand an applied pressure exerted by the fluid (e.g., in a filter). Some applications may require a low thermal expansion coefficient (CTE), good thermal shock resistance, or good thermal shock damage resistance.
In many applications, channels or other substantially “open” regions of the body (offering minimal impedance to fluid flow) are used to substantially increase area for reaction or filtration (for example, as in U.S. Pat. Nos. 4,253,992 and 4,276,071). In such applications, relatively thin walls separate regions having substantially minimal impedance to fluid flow. Walls separating the channels should have both high porosity to maximize surface area or permeability, but not so high porosity that mechanical properties are degraded, and the pore size distribution should provide for the desired treatment (e.g., actually filter the species being removed).
In some applications (e.g., a filter bed) a body may be essentially hydrostatically supported during operation, and so require little mechanical strength (e.g., shear or tensile strength) during filtration. Some applications also include backwashing, which often creates a different mechanical stress than that created during filtration. In such instances, some mechanical strength or appropriate containment may be necessary. Thermal stresses, thermal expansion mismatch, changes in crystallographic structure, physical impact, and other factors may also create certain requirements of a body in a given application.
Increasingly, cost may be an important factor in a given application. Costs may include capital costs associated with fabricating the body and associated fluid control system itself. Cost may also include operational costs. Costs may also include disposal costs, environmental costs, “cradle to grave” costs and other costs associated with implementing a particular treatment solution. The energy required to create and implement a particular body may be an important cost factor, and in such cases, reducing the energy required to make and use a particular device may be advantageous. Cost may include a cost associated with emitting global warming gases, environmental pollutants, or other contaminants. Often, minimizing the embodied energy associated with a product (e.g., the energy required to create and implement the product) and/or minimizing a total lifecycle cost of the product may be advantageous. The implementation associated with a treatment method, the method of treating the fluid with the body, the disposal of treated substances and/or the body itself, and other lifecycle costs may generally include both capital costs (including raw materials costs), operational costs (including lifetime) and disposal/recycling costs.